In many applications--e.g. automatic gain control (AGC) systems in audio and TV apparatus--the signals whose amplitudes may vary within a wide range need to be amplified so that an amplified signal with constant amplitude can be presented at the output.
In such automatic gain controls as usually comprise at least one variable gain amplifier in a loop structure, a signal level detector, a control current generator driven by the signal level detector, and a stabilizing capacitor, it is indeed the variable gain amplifier that constitutes the most critical part of the arrangement.
For a truly effective control of the gain, the dependence of the variable gain amplifier gain on control voltage should follow an exponential law and this dependence should remain stable as temperature varies.
The most widely used arrangement in the art for implementing a variable gain amplifier is to cut down the gain of a balanced stage using a Gilbert multiplier in the manner shown in FIG. 1.
The signal to be amplified is applied to the input terminals of a differential stage which includes transistors Q1 and Q2.
Further, a control voltage Vcontrol is applied to the input terminals of two differential structures Q3, Q4 and QS, Q6 of a Gilbert multiplier, connected to the legs of the differential stage. The control voltage Vcontrol determines the amplification gain of the signal that appears at the output terminals formed by one of the leg pair of each of the two differential structures.
The problems brought about by this prior approach are the following:
--the gain dependence on the control voltage is not exponential but rather of the "hyperbolic tangent" type, and PA0 --the noise introduced by the multiplying cell is high, and a common output mode adjusting circuit must be used.
In order to obtain an exponential type dependence from the control voltage for the amplifier gain, a complicated distortion of the control signal must be resorted to, so that the exponential type dependence is difficult to stabilize when temperature changes.
Another method known to those skilled in the art for providing a variable gain amplifier is to use as gain control a field-effect (FET) M.sub.10 transistor acting as a variable resistor, in conjunction with load resistors R.sub.8, R.sub.9, transistors Q.sub.11 and Q.sub.12, and current generators G.sub.12 and G.sub.13, as shown in FIG. 2.
In such a case, the gain relation to the control voltage follows a quadratic law, and the drawback is even more serious since a limited range of gain values is available, thereby making the cascade connection of at least two stages necessary to expand that range.
A further drawback is a limited input dynamic range.
Similar to the previously mentioned method, any possible exponential dependence obtained by skewing the control voltage would be difficult to stabilize for temperature changes.